1. Field of the Invention
The present invention relates generally to a method and apparatus for data transmission in a mobile communication system, and more particularly to a method and apparatus for transmitting and receiving packet data between a UE and a Node B using a Hybrid Auto Repeat Request (HARQ) in a mobile communication system utilizing an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
For next generation mobile communication systems requiring a high data transfer rate, research is currently being conducted to provide various quality services to users. In order to provide wireless multimedia services of high speed and high quality, broadband spectrum resources are needed. However, by using the broadband spectrum resources, severe fading on the transmission path is caused by multi-path propagation, and frequency selective fading occurs within the transmission band. Therefore, a communication system using multi-carriers robust to frequency selective fading (hereinafter, referred to as a “multi-carrier communication system”) is widely used for high speed wireless multimedia services. The multi-carrier communication system is a communication system that utilizes a modulation scheme using multiple sub-carriers. A typical example of such a modulation scheme is an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
The OFDMA scheme corresponds to a multiple access scheme based on the OFDM scheme. In the OFDMA scheme, a portion of all sub-carriers is reconfigured into a sub-channel, which is in turn allocated to a specific user terminal. The sub-channel is a channel including at least one sub-carrier. By using the OFDMA scheme, it is possible to perform dynamic resource allocation through which a sub-channel can be dynamically allocated to a specific user terminal according to the fading characteristic of a wireless channel.
Further, in the OFDMA scheme, a multi-user diversity gain increases with as the number of subscriber terminals increases, i.e., with an increase in the number of users, and thus, active research on the OFDMA scheme is particularly aimed at the next generation communication systems requiring a relatively high transfer rate.
In order to efficiently allocate each sub-channel to a multi-user, the OFDMA communication system also uses a scheme for temporally dividing the sub-channel. When temporally dividing the sub-channel, each divided time unit is called a Transmission Time Interval (TTI).
Hereinafter, a Long Term Evolution (LTE) system, which is one of the next generation mobile communication systems using the OFDM, will be described by way of example. The 3GPP (3rd Generation Partnership Project) is now discussing the LTE system as a next generation UMTS (Universal Mobile Telecommunication Service) system. The LTE system, which is hoped to be commercialized by 2010, enables a bandwidth, which has been limited to 5 MHz in the existing 3rd generation mobile communication, to vary from 1.25 MHz to 20 MHz, and is currently under standardization for implementing a data transfer rate of about 100 Mbps.
FIG. 1 illustrates a communication network structure of a conventional LTE system. In the LTE system, User Equipments UE #1 141, UE #2 143, and UE #3 145 are connected with a core network 110 through an Enhanced Node B (ENB) 133 and an access gateway 120. A UE is also commonly referred to as a mobile station (MS), a terminal equipment (TE), or the like, but is expressed by “UE” in FIG. 1 because such an expression is customary. Further, although three UEs are illustrated in FIG. 1, it is obvious that the number of UEs may vary according to communication environments.
FIG. 2 illustrates a conceptual view for explaining a transmission resource block in the LTE system. In FIG. 2, it is assumed that transmission resources used in the LTE system include a frequency band with an overall bandwidth of 5 MHz 205, the frequency band is divided into 25 sub-channels with a size of 180 KHz 210, and each sub-channel is temporally divided in units of TTIs with a size of 1 msec 215. Accordingly, transmission resources of the LTE system can be represented by dividing the overall frequency band into a plurality of sub-channels and dividing each sub-channel in units of TTIs. Each of the smallest rectangles illustrated in FIG. 2 corresponds to a minimum unit of transmission resources in the LTE system, and is called a “resource block (RB)”.
Hereinafter, a description will be given of HARQ. The HARQ is a combination of a Forward Error Correction (FEC) scheme and an Automatic Repeat Request (ARQ) scheme, which are typical transmission error control technology used in packet data transmission systems.
Assuming that a UE has transmitted a packet to an ENB, the ENB attempts to perform error correction for an HARQ packet transmitted by the UE, and determines whether or not to request the UE to retransmit the HARQ packet by using a simple error detection code, such as a CRC (Cyclic Redundancy Check) code. More specifically, if there is no error in the received HARQ packet, the ENB transmits an HARQ ACK signal to the UE. However, if the received HARQ packet is erroneous, the ENB transmits an HARQ NACK signal to the UE. In response to this, the UE transmits a retransmitted HARQ packet corresponding to the HARQ NACK signal to the ENB, and transmits a new HARQ packet corresponding to the HARQ ACK signal to the ENB. Also, when any HARQ packet is erroneous, the ENB can increase a data reception rate by using soft combining of previously received packets and a retransmitted packet.
Reference will now be made to a scheme in which an ENB allocates transmission resources to UEs by using the HARQ scheme. If a UE is allocated one transmission resource (i.e., one RB) at any point of time, it uses the allocated transmission resource until packet transmission thereto is completed using the HARQ scheme. Also, once the packet transmission to the UE is completed, the ENB allocates the transmission resource to another UE.
When packet transmission is erroneous in a data communication system using the HARQ scheme, a data reception rate in a receiving UE theoretically increases with an increase in the number of times of HARQ packet retransmission. However, because a delay requirement is given according to the types of communication services, a maximum retransmission limit of HARQ packet data retransmission is limited based on the types of transmitted data according to communication services. The delay requirement refers to a maximum permission delay time allowable within a range in which it will not affect a communication service if there is a transmission delay during transmission/reception of one packet.
For example, for interactive gaming, a maximum permission transmission delay amounts to merely several tens of milliseconds because a user is inconvenienced by any delay in data transmission. Thus, a maximum retransmission limit of an HARQ packet is limited to a small number of times, for example, one time. Conversely, for an FTP (File Transfer Protocol) service, a maximum permission transmission delay may amount to several seconds because it will not affect the service if a transmission delay time lengthens. Consequently, for a communication service tolerable to a transmission delay, retransmission may be performed a large number of times, for example, retransmission may be performed up to 15 times.
In principle, an ENB must efficiently allocate limited transmission resources to corresponding UEs. Therefore, in a data communication system using the HARQ scheme, an ENB ideally allocates a transmission resource to a UE only when needed.
For example, a transmission resource needs to be used until an ENB completes receiving a packet transmitted by a UE, and when a UE cannot receive ACK from an ENB within a maximum retransmission limit due to transmission errors.
In the latter case, the ENB cannot know if the current number of times of retransmission reaches the maximum retransmission limit. That is, when a UE fails to receive ACK from an ENB, even beyond the maximum retransmission limit, there is a problem in that the ENB cannot allocate a transmission resource to another UE although the UE does not use the corresponding transmission resource. Also, the maximum retransmission limit of an HARQ packet varies according to the types of transmitted data, as mentioned above, but an ENB cannot know what type of data is included in an HARQ packet until it successfully receive the HARQ packet. Therefore, there is a need for a method that enables an ENB to know what type of data is included in an HARQ packet and to know a maximum retransmission limit that is applied to the HARQ packet. In summary, there is a need for a method for an ENB to know if a current packet received from a UE corresponds to a maximum retransmission limit and what is the type of the data packet.
FIG. 3 illustrates a call processing diagram for explaining a method of transmitting and receiving packet data between an ENB and a UE using the HARQ scheme in a conventional mobile communication system. If the ENB 305 grants any transmission resource to the UE 310 in step 315, the UE transmits a data packet through the transmission resource in step 320. For reference, a MAC PDU (Medium Access Control Packet Data Unit) is illustrated as an example of a data packet in FIG. 3. In step 325, the UE transmits a last transmission indicator (LTI) along with the data packet. Here, the LTI is an external control signal indicating whether or not the packet transmission corresponds to the last retransmission, and is set to “No”.
In step 330, if the ENB receives the packet, and determines through a CRC operation that the packet is erroneous, it transmits a NACK signal to the UE. In step 335, the UE retransmits the packet, using the same transmission resource at a point of time 375, a given period of time (hereinafter referred to as “HARQ RTT”) later than a point of time 365 when the packet is transmitted.
Assuming that a maximum retransmission limit of the packet is set to 2, in step 340, the UE sets the LTI to “No” again, and transmits it along with the packet. Then, the ENB successfully receives the packet, and can perform soft combining of the packet and previously received packets.
In step 345, if the ENB determines through a CRC operation that the packet retransmitted in step 335 is still erroneous, it transmits a NACK signal to the UE in step 345. In step 350, the UE retransmits the packet once again by using the same transmission resource at a point of time an HARQ RTT later than a point of time when the packet is retransmitted. The retransmission in step 350 is the second retransmission following the retransmission in step 335. Thus, in step 355, the UE sets the LTI to “Yes”, and retransmits it along with the packet. On receiving the packet with the LTI set to “Yes”, the ENB transmits ACK or NACK to the UE according to a CRC operation in step 360. Because the LTI is set to “Yes” in step 355, in step 380, the ENB recognizes that the transmission resource allocated to the UE is not used any more irrespective of a result of the CRC operation in step 360, and reallocates the transmission resource to another UE.
In the conventional solution illustrated in FIG. 3, there is a problem in that transmission resources cannot be efficiently used because a UE uses an additional transmission resource for transmitting a separate external control signal (i.e., LTI) to an ENB. As in generally known in the art, the transmission resource used for transmitting the external control signal is several times or several tens of times as large as a transmission resource used for transmitting a packet. Therefore, in order to efficiently use transmission resources, there is a need for a method for a UE to inform an ENB whether or not a maximum retransmission limit is reached, even without using a separate transmission resource.